The exhaust gas from an internal combustion engine, such as an automobile engine, contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and the like. These substances can be purified by an exhaust gas purifying catalyst capable of oxidizing CO and HC and at the same time, reducing NOx. As a representative exhaust gas purifying catalyst, a three-way catalyst comprising a noble metal, such as platinum (Pt), rhodium (Rh) and palladium (Pd), and a porous metal oxide support, such as γ-alumina supporting the noble metal, is known.
However, such a noble metal is expensive and may be in short supply in the future due to a reduction in reserves. Accordingly, use of a perovskite-type solid solution, as a catalyst other than a noble metal, is proposed as a catalyst for purifying an exhaust gas.
Regarding the perovskite-type solid solution as a catalyst, Japanese Unexamined Patent Publication No. 6-106062 proposes an exhaust gas purifying catalyst designed to have a high exhaust gas purifying performance even at low temperatures such as idling time by using a perovskite-type solid solution as the core and disposing, on the core, a perovskite-type solid solution having a noble metal solid-dissolved therein.
As for the base metal-based exhaust gas purifying catalyst, it is also known to use iron oxide.
Japanese Unexamined Patent Publication No. 2005-296735 discloses a catalyst obtained by loading iron oxide on a support containing a ceria-zirconia solid solution, wherein the support has 0.1 mL/g or more of pores having a pore diameter of 3.5 to 150 nm. Japanese Unexamined Patent Publication No. 2005-296735, describes that, in such a catalyst, the iron oxide acts as an active site.
Japanese Unexamined Patent Publication No. 2004-160433 proposes a technique of forming a solid solution of iron and at least one metal selected from the group consisting of cerium, zirconium, aluminum, titanium and manganese, so as to utilize the oxygen storage capacity (OSC) of iron, which is attributable to a change in valence between Fe2O3 and Fe3O4. Also, in Japanese Unexamined Patent Publication No. 2004-160433, it is indicated that in the case of using this solid solution as a catalyst, HC and CO can be oxidized by allowing the iron to act as an active site.
In Japanese Unexamined Patent Publication No. 2004-160433, a solid solution concretely shown is only an iron oxide-titania solid solution, and it is stated that the iron oxide-titania solid solution provides OSC through the following reactions:2Fe2O3+4TiO2←→4FeTiO3+O2  (1)6Fe2O3+12TiO2←→4Fe3Ti3O10+O2  (2)3Fe2O3+6TiO2←→Fe6Ti6O12+O2  (3)
As described in these publications, it is known to use a base metal (e.g., iron) as a catalyst metal for an exhaust gas purifying catalyst. However, particularly, a catalyst using iron as a catalyst metal has a problem that the oxidation state of iron changes during use of the catalyst, and the catalyst deteriorates due to change in the iron volume. In the case of loading iron at a high concentration on the surface of a ceria-zirconia solid solution as described in Japanese Unexamined Patent Publication No. 2005-296735, there is a problem that the surface area of the catalyst decreases and causes deterioration in the performance of the catalyst.
Accordingly, an object of the present invention is to provide an exhaust gas purifying catalyst, which solves conventional problems regarding use of iron as a catalyst metal and has good OSC.